DAMAGE TO THE DISCS
Intervertebral discs are not easily injured. The gel-like nucleus of the disc allows it to change shape, rather like a cushion that is sat on, in response to pressures that are exerted on it. The tightly woven fibres of the outer casing, the annulus, are very strong and moderately elastic, so that, like a cushion cover, it is able to accommodate, without tearing, most changes in the shape of its contents. However, although strong, it is not invulnerable. It may tear if subjected to a twisting action, that is, any movement in which the vertebrae above and below the disc are made to rotate in opposite directions. This sort of injury can be caused by an untoward movement in the course of vigorous exercise, for instance, or through lifting a heavy object awkwardly. It is most likely to happen when the trunk is bent forward: in this position the lumbar facet joints are less effective at preventing rotation.
When the tear is very severe, the pulpy nucleus of the disc is forced out by the pressure of the vertebrae above the tear (like toothpaste squeezed out of a tube), and protrudes out of the tear. Such an injury is rare. If the outer casing does not tear, the nucleus will remain contained within the casing, but its pressure may cause the disc to bulge outwards.
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Tags: Healthy bones